COMPRESSION AND PRESSURE-INDUCED AMORPHIZATION OF CO(OH)2 CHARACTERIZED BY INFRARED VIBRATIONAL SPECTROSCOPY

The infrared-active (A2u) O-H vibration of Co(OH)2 decreases in frequency under hydrostatic compression to 51 GPa at 290 K. Similarly, the bond anharmonicity, determined from the nu1-->nu2 absorption-band difference, increases by more than a factor of 2 between 0 and 20 GPa. Both changes are attributed to an increase in the O-H bond length due to enhanced hydrogen bonding under pressure. The full width at half maximum (FWHM) of the fundamental absorption band increases abruptly by approximately 100 cm-1 at 11.2 (+/- 0.3) GPa, and continues to increase at a rate of approximately 3.3 cm-1/GPa up to 36 GPa. Above 36 (+/- 2) GPa and below the onset of amorphization, the FWHM changes at a slower rate, 0.8 (+/- 0.1) cm-1/GPa. The abrupt change in FWHM is reversible on decompression, and is interpreted in terms of a pressure-induced crystal-to-glass transition exhibiting a small hysteresis compared to similar compounds. The rapid variation in FWHM above the transition pressure suggests that the amorphous structure is continuously modified between 11.3 and 36 GPa.